How to move freestyle line sets to another layer?
Sadly this is not possible by default, however here is an effective workaround



Under the Render Layers tab there is a checkbox that enables/disables it for that layer. And no, this does not render the Freestly pass if you just set it by itself on a new render layer.



Using this we will be able to (with come complex compositing) get just the Freestyle pass.

Start by adding a new Render Layer and unchecking the Freestyle checkbox. When you render now, you should get two images, one with freestyle and one without. This is exactly what we need for the next step, which will be done in the compositor.

In the compositor, add another Render Layer node by selecting the already present one and pressing Shift+D. Change the dropdown on the bottom of it to the second render layer. Now add a Color Mix node and set it to Subtract and connect it as shown.



Now we have a basic image we can work with:



Now replicate this node setup to take this image and make it useable:



(Basic Math node set to Multiply with a value of 10, and a color mix node set to black and white with the math node input driving the factor.)

Now we have our image to use as the factor to mix the rendered image with a black (or any other) color. Add another color mix node, connect the output of the nodes we just added to the factor input, change the top color to black, and connect the plain render layer image output to the bottom color input. You should now have a node setup like this:



(I added the blur node to make the Freestlye lines less sharp, but it is completely optional)

Your render should now be complete, you can now access just the freestlye render:



And can use that to customize how it is used in your final image.